1. Field of the Invention
This invention relates to conductive compositions which are solderable and flexible, and to their method of production. More particularly, this invention relates to conductive compositions that exhibit good flexibility and solderability and that can be bonded directly to substrates.
2. Description of the Prior Art
An electrically conductive composition applied to a supporting or base structure must be sufficiently conductive to carry electric current and must be firmly adhered or bonded to the support or base. Furthermore, it is desirable that the conductive composition be directly solderable and flexible.
U.S. Pat. No. 3,030,237 discloses an improved coating material essentially comprised of organic resin carrier and a suitable metallic pigment. The invention lies in the discovery that a coating having a metallic pigment component consisting of a mixture of ball and flake particles having particular dimensions and used in certain proportions, in combination with an organic resin carrier, can exhibit good adhesion and electrical conductivity. Also disclosed is that conventional wire leads may be silver-soldered to the cured coating using entirely common silver soldering techniques.
U.S. Pat. No. 2,959,498 discloses a method of forming conductive silver circuits by first applying to a resinous dielectric surface a layer of an incompletely cured thermosetting resin, and then applying on said layer, in the outline of the desired conductive circuit, a layer of finely divided silver admixed with a thermoplastic resin containing a solvent for both of said resins, and then heating to cure both layers of applied resins. It is essential that the resinous panel first be coated with a thermosetting resin and that this resin is only partially cured before application of the conductive composition. The conductive circuit is applied over the undercoating, preferably by squeezing a silver paste in the desired electrical circuit pattern. The silver paste is composed of finely divided silver particles in a thermoplastic resin in a ratio of between 4:1 and 10:1 silver to resin. The resulting silver circuit pattern is firmly adhered to the resinous substrate and the silver lines readily accepts solder.
U.S. Pat. No. 4,371,459 discloses screen printable conductor composition that is flexible comprising (a) a conductive phase containing silver and base metal powders dispersed in a solution of (b) a multipolymer prepared by copolymerization of vinyl acetate, vinyl chloride, and an ethylenically unsaturated dicarboxylic acid and a linear aromatic polyester resin dissolved in (c) volatile nonhydrocarbon solvent. These compositions are particularly useful for application to membrane touch switches.
U.S. patent application Ser. No. 537,740 filed Sept. 30, 1983, now U.S. Pat. No. 4,564,563, discloses a solderable electrically conductive composition comprising metallic silver particles embedded in a matrix formed from acrylic, carboxylated vinyl and an epoxy. The composition is formed by dissolving acrylic powder and vinyl powder in respective solvents to form a first solution and a second solution. The solutions are then mixed with metallic silver particles and an epoxy to form an ink which is applied to a substrate to form a film thereon. The film is cured to evaporate the solvents and allow polymerization to occur thereby having a solderable electrically conductive film.
Heretofore, conductive compositions developed for use in connection with supporting or base structures or other electrical equipment have been characterized by unacceptably high electrical resistivity or unacceptably low adhesion to the supporting or base material. Furthermore, many prior art compositions suffer from the deficiency of not being directly solderable; that is, the time consuming and expensive process of electroless plating or the like must be employed in order to apply solder to the conductive composition. Yet another deficiency lies in the fact that many of the prior art conductive compositions are not flexible and can therefore not be applied to flexible substrates. Additionally, many of the prior art compositions cannot be bonded directly to the substrate on which they are applied.
While some prior art compositions do solve some of these deficiencies they are deficient in that complicated formulations employing additional compounds must be used and additional processing steps must be conducted. Furthermore, none of the prior art compositions exhibit all of the characteristics of sufficient conductivity, flexibility and solderability while at the same time being capable of direct bonding to substrates.
It has been a long sought goal to provide conductive compositions that have good conductivity, adhesion and flexibility and which are directly solderable without having to resort to the need for complicated procedures or formulations. It has also been desired to have a conductive composition that has sufficient conductivity and flexibility to make it capable of replacing mechanical end connectors on flexible polymer thick film circuits.